1. Field of the Invention
The present invention relates to a sidelobe canceller wherein an array of auxiliary antennas is provided in addition to a main antenna for cancelling an undesired signal introduced to the main channel signal by the sidelobes of the main antenna.
2. Description of the Related Art
A prior art sidelobe canceller consists of a main antenna which is oriented to receive a desired signal and an array of auxiliary antennas. A plurality of multipliers are connected to the auxiliary antennas for weighting the outputs of the auxiliary antennas with controlled weight values. If a jamming signal, uncorrelated with the desired signal, is present in the sidelobes of the main antenna the quality of transmission is severely degraded. To provide sidelobe cancellation, the weighted signals are summed to produce a sum signal which is subtracted from the output signal of the main antenna. By using the sidelobe cancelled signal as a reference, the weights of the multipliers are updated so that the auxiliary antennas orient the main lobe of their directivity pattern toward the jamming signal source. Under this condition, the sum signal represents a replica of the jamming signal. The least mean square algorithm and the Applebaum algorithm are known in the art to derive weight coefficients. The Applebaum algorithm is one which derives the weight coefficients by introducing a steering vector to the LMS loop of the sidelobe canceller for estimating to some extent the direction of arrival of the desired signal. The weight control provided by the Applebaum algorithm maximizes the ratio (SINR) of desired to undesired signal level (interference signal plus noise).
An adaptive equalizer is used for adaptively equalizing intersymbol interference caused by a multipath fading channel. If the adaptive equalizer is used in combination with the prior art sidelobe canceller and if the time difference between the paths of the multiple fading channel is small, there is a shift in fade pattern from frequency selective fading to flat fading and the desired signal itself will be lost. This problem cannot be solved by the use of the adaptive equalizer and diversity reception would be required. In addition, since the output signals of the auxiliary antennas also contain a desired signal component, the sum signal contains it as well as the replica of the undesired signal. The sidelobe cancelled signal would severely decrease in amplitude as a result of the subtraction of the desired component from the main antenna when they are under a certain amplitude and phase relationship.